Simultaneous magnetic and chemical order-disorder phenomena in Fe3Ni, FeNi, and FeNi3

We study the magnetic and chemical order-disorder transitions in face-centered-cubic FeNi3, FeNi, and Fe3Ni, by Monte Carlo (MC) simulations using the Ising approximation. The calculations are done: (1) with magnetic interactions only, assuming fixed preset degrees of chemical order, (2) with chemical interactions only, and (3) with both magnetic and chemical interactions acting simultaneously. As expected and known from measurements, the degree of chemical order is found to have a large influence on the magnetic transitions. On the other hand, although one might expect the effects of magnetism on the chemical ordering processes to be small (because the chemical bond energies are much larger than the magnetic exchange bond energies), one finds that the latter effects are also large. Several new features arise that are not predicted by mean-field theory or MC simulations with chemical interactions only. For example: (1) chemical order can be induced where using chemical interactions only leads to the prediction of no chemical order, (2) chemical segregation can be induced where using chemical interactions only leads to the prediction of no chemical segregation, (3) FeNi3 and Fe3Ni are found to have significantly different chemical ordering temperatures where chemical interactions only lead to equal ordering temperatures, (4) chemical ordering temperatures are significantly shifted from their chemical interactions only values, even when the chemical ordering temperature is larger than the magnetic ordering temperature (or Curie point), (5) abrupt steps can occur in the spontaneous magnetization at the chemical ordering temperature, when the latter is smaller than the magnetic ordering temperature, and (6) nonlinear relations arise between the chemical ordering temperature and the usual differential bonding parameter U=2U(FeNi)-U-FeFe-U-NiNi, where the U-ij's are the near-neighbor pair-wise chemical bonds.